1. Field of the Invention
The invention relates to center break switches, such as for electrical power substations and transmission lines, and particularly to such a switch in an arrangement facilitating opening of the switch.
2. Background Art
Center break switches have (in a single pole) two switch blades with mating contacts that meet, and separate, between a pair of rotatable blade supports. In a common type, the blade supports include ceramic or polymer insulators that are generally cylindrical with lateral sheds. The supports are joined at one end (nominally, the “bottom”) to a quite rigid metal base with a bearing for rotation of each support relative to the base and a mechanism for imparting rotational force to both supports, hence moving the switch blades into or out of a closed contact position. The supports, and their axes of rotation, are substantially parallel to each other in one switch type or, in another type, are in a substantially V-shaped configuration. Switches of interest include those described and illustrated in Cleaveland/Price Inc. descriptive bulletin DB-126A02, “Aluminum Center Break Switch”, published in 2002, that is representative of prior art to the present invention.
Operation of such switches is in some cases manual (e.g., by a handcrank or a swing handle) and in some cases by electric motor. Whether manually or motor operated, it is desirable to operate the switch easily and quickly with only modest requirements on the equipment and personnel. For example, some switches have a handcrank operator for manual operation. The handcrank is connected to the rotatable support apparatus through a gearbox with a gear ratio typically in a range from about 10:1 to 40:1, as specified by a user. A higher gear ratio allows a switch to be opened with less manual force but requires more time, which is generally undesirable.
Another factor in switch operation is that a typical installation has three poles, substantially alike, one for each phase of a three phase electrical system, and the operator must apply sufficient force to operate all three poles together. A maximum operating force, for three-phase switches, is typically specified to be in a range from about 35 to 70 pounds. Some installations have switches ganged together in even larger numbers, such as six poles with two poles for each phase of a three phase system.
Center break switches are now applied over a wide range of voltages, including high voltage systems up to a nominal rating of at least about 230 kV. Required switch size increases with increasing voltage (for contact clearance when the switch is open and for sufficient distance across the insulative supports) so that the rotatable supports and the contact blades reach up to several feet in length. This makes for a relatively massive structure to be moved and the longer supports make them more subject to bowing that can affect operation. In general, however, considerations affecting the opening force requirements apply to some degree regardless of the switch size or the number of switches operated together.
Switches operate in a variety of environments including those that can, particularly with age, change the amount of required operating force. One type of known switch has contacts with engaging surfaces that meet substantially in a horizontal plane like that of the arcuate motion of the contacts resulting from blade supports’ rotation. This produces considerable wiping action between the contacts during opening and closing that helps keep the contact zone free of debris and oxides. In this respect, sliding friction between the contacts enhances switch performance while also having an influence on the required opening force.
A variety of contact configurations are used in various center break switches. For example, some have appreciable contact engagement in a vertical plane that is substantially perpendicular to the plane in which the blades move. Still, in any of the contact configurations, there is some degree of sliding friction that can affect switch opening. Prior art has largely relied on a basic assumption that the axes of rotation of the insulative supports are substantially fixed. While prior center break switches have been generally successful, their design has not addressed the fact that sliding friction between the contacts during a switch opening can alter the location of the axes of rotation of the supports, particularly, but not limited to, those of larger units. Altering the axes of rotation by contact friction results in greater required force and time to separate the contacts than if those axes were fixed.